1. Field of the Invention
The present invention relates in general to high-pressure small metal vapor discharge lamps. More specifically, the invention relates to high-pressure small metal vapor discharge lamps which are lit by a power supply with no polarity alteration such as direct current.
2. Description of the Prior Art
In recent years, in view of energy saving, it has been promoted to develop metal vapor discharge lamps such as, for example, metal halide arc lamps.
Since metal vapor discharge lamps have superior luminous efficiency compared with incandescent lamps, the former tends to be used in place of the latter. These metal vapor discharge lamps are usually lit by the power supply for example A.C. 120 V, 60 Hz. The electric power is fed to metal vapor discharge lamps through a ballast, which is generally installed separated from metal vapor discharge lamps. When considering them as replacements for the incandescent lamps which are mostly used for room lighting in general households and shops, etc., the essential requirements are to incorporated the ballast with the lamp and, furthermore, to make the ballast small, light-weight and low-cost. However, it is difficult to satisfy the conditions for the ballasts in general use which employ choke coils. Recently, through the development of transistors, IC, etc., it has become possible to construct an electronic circuit as a ballast which can satisfy the conditions described above. Although the direct current lighting method and the high-frequency lighting method can be considered as such electronic circuit systems described above, if employing the high-frequency lighting method, the phenomenon called acoustic resonance occurs in particular frequency bands and the arc wavers so that this becomes a cause of extinction of a lamp.
In particular, in the case of metal halide lamps, the high-frequency lighting method is unsuitable since the frequency band in which acoustic resonance occurs is very broad through the influences of the shape of the luminous tube and of the fillers. Therefore, as an electronic ballast for metal halide lamps, a lighting method using direct current power source is particularly desirable.
In the course of development of metal vapor discharge lamps, such as metal halide lamps, which use direct current power source, the inventor discovered that, when discharge lamps which were designed for conventional alternating current lighting use with electrodes having coils wound round the tops of the electrode shafts were lit by direct current power source, there were many lamps which failed to light up because devitrification and cracks occurred in the luminous tube wall in the vicinity of the cathode and so the luminous tube leaked the filler.
Furthermore, it was proved that the phenomenon described above becomes more remarkable with small lamps, such as those of less than 100 W, in which the cathode and the wall of the luminous tube are closer to one another.
The causes of the above-described phenomenon were found when further comparative observation was carried out with lamps for alternating current lighting. When a lamp was lit by direct current power source, arc spot was generated at the seal end of a cathode even if the lamp was stable in the normal condition, and there were times when no arc spot moved to the top of the cathode. It caused the lamps to be cracked in almost all cases if the lamps kept on the state for a long time in the above-described condition.
Conversely, in the case of lighting by alternating current, although the discharge commenced from the seal end of the electrode immediately after starting, in every lamp the arc spot moved to the top of the electrode in a short time and cracks did not occur. It was assumed that this kind of phenomenon was caused by the following factors. That is to say, for both the cases of alternating current and of direct current, since the condition immediately after starting is one of a low pressure of less than 1 atmosphere, the discharge commences in a condition where the discharge distance is longer.
However, as time elapses, the temperature in the luminous tube rises and the pressure in the luminous tube also rises. There is a high pressure of more than 1 atmosphere at the rated lighting. For instance, in the case of metal halide lamps, the pressure rises to about 10 atmospheres or even more. Therefore, in order to maintain a stable discharge, the arc spot moves from the electrode seal end to the top of the electrode, in other words, it moves in a direction which makes the discharge distance d smaller in order to satisfy the well-known law Pd=const. (P is pressure, d is discharge distance). With regard to this phenomenon, in the case of alternating current, since both electrodes repeat the operations of the cathode and the anode in turn each half cycle. When both electrodes act as anode in turn, the tops of the electrode are heated in turn by the arc concentrating on the whole electrode so that the arc easily moves to the top of the individual electrode with the pressure increase. On the contrary, in the case of direct current, the arc becomes a spot at the cathode side and concentrates on only a very limited portion of the electrode. Therefore, only the portion where the arc is concentrated is heated. Moreover, since the coil portion of the electrode acts as a heat radiation fin, even if the pressure in the luminous tube rises, the temperature of the top of the electrode does not rise sufficiently for emitting electrons. Furthermore, since there is no polarity reversal, it is assumed that the movement of the arc from the position where it has once been a spot is not occur unless there is some trigger.
Therefore, when an arc spot occurs at the seal end of the cathode and does not move to the top of the cathode, the high temperature arc has been positioned close to or contact with the inner surface of the luminous tube for a long time, this causes devitrification and cracking of the wall surface of the luminous tube. Furthermore, the fact that the arc spot is generated at the seal end or the top of the cathode in different cases means that the respective arc lengths differ. Therefore, since each lamp voltage differs from one another in correspondence to the difference of the arc distance described above, there is inconvenience that each lamp voltage may be not constant at every lighting.
Japanese Patent Application Ser. No. 123,431 filed July 8, 1983 (Laid open No. 85-17849) in the name of Shinji Inukai and entitled SMALL METAL VAPOR ARC LAMP discloses one of the solutions of the problems described above. As can be seen in FIG. 1, a cathode 1 includes an electrode shaft 2 and a coil 3 which is wound around the top portion of electrode shaft 2 and extends therefrom. A hollow portion 4 is defined within coil 3.
According to the above-described constitution, since the heat capacity of the top portion of coil 3 is small because of hollow portion 4, the temperature of the top portion of coil 3 rises rapidly to the temperature at which electrons are easily emitted. Therefore the arc spot produced on cathode 1 quickly moves to the top of cathode 1 thus preventing devitrification and cracking of the wall surface of the luminous tube. Hollow portion 4, however, causes the arc spot to be fluctuated thus flickering occurs.
Japanese Patent Application Ser. No. 135,174 filed July 26, 1983 (Laid open No. 85-28155) in the names of Shinji Inukai and toshihiko Ishigami and entitled SMALL METAL VAPOR ARC LAMP discloses another solution of the problems. As can be seen in FIG. 2, a bar-shaped element 5, made of high melting-point metal, is inserted into the other side of coil 3. Tops of electrode 2 and bar-shaped element 5 are arranged apart from one another so that a hollow portion 4' is established within coil 3.
This prior art can prevent the fluctuation of an arc spot as well as the devitrification and cracking of the luminous tube. However, since the entire length of coil 3 on cathode 1 of a high-voltage small metal vapor arc lamp is very small, e.g. about 2 mm, it is rather troublesome to provide hollow portion 4' of a prescribed length in such a small coil. There are also disadvantages in yield rate and operations efficiency. These disadvantages cause a manufacturing cost to be increased.
Japanese Patent Application Ser. No. 110,860 filed June 22, 1983 (Laid open No. 85-3846) in the names of Shinji Inukai, Yasuki Mori and Akihiro Inoue and entitled SMALL METAL VAPOR ARC LAMP discloses another solution of the problems. In FIG. 3, cathode 1 is composed of an elongated element made of high melting-point metal such as tungsten. Cathode 1 has no coil. This prior art achieves the same effects as other prior arts described above.
Generally, in discharge phenomena, it is desirable that heat capacity of a portion of an electrode where an arc occurs is as small as possible to accomplish transition from glow to arc smoothly. On the contrary, it is desirable to have a large heat capacity to prevent an electrode from melting accompanying the temperature rise of an electrode, when an arc discharge has occurred. The melting of an electrode concerns a lamp voltage increase related to a lamp life, and an arc extinction.
When a cathode is composed of an elongated element as described above, a lower limiting value of an electrode shaft diameter is determined in view of the prevention of melting of the electrode. An upper limiting value is determined by the boundary point at which transition from glow to arc occurs. Furthermore, even in the area where transition from glow to arc occurs, it is desirable to accomplish the transition smoothly in order to improve lumen maintenance factor as well as to decrease sputtering of an electrode. Further improvement of these points has been desired.